Disk brake with galvanic corrosion protection



NOV. 4, 1969 c. m 3,476,222

DISK BRAKE WITH GALVANIC CORROSION PROTECTION Filed March 27, 1968 Hans C; Klein INVENTOR.

5S W ttomey United States Patent 3,476,222 7 DISK BRAKE WITH GALVANIC CORROSION PROTECTION Hans Christof Klein, Hattersheim (Main), Germany, as signor to Alfred Teves G.m.b.H., Frankfurt am Main, Germany, a corporation of Germany Filed Mar. 27, 1968, Ser. No. 716,507 Claims priority, application (7;;rmany, Apr. 1, 1967,

T Int. Cl. B61b 17/ 18; F16f 65/84 US. Cl. 188-152 6 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to protective closures for open-ended cylinders and, more particularly, to a shield which protects the pistons movable in such cylinders against corrosion.

Dust caps and similar closures according to prior art, which are designed to close oif open-ended cylinders and especially fluid-responsive cylinders of the type used in automotive systems have not proved to give generally satisfactory protection against corrosion, because the material from which they are made and their method of attachment represent compromise solutions for a diflicult problem. The seals must be effective at temperatures fluctuating between extreme heat and cold and under exposure to a variety of solid and liquid, organic and inorganic chemicals such as, for example, the salts used to treat road surfaces in winter, the deliquescent materials deposited on dusty roads during dry seasons and similar naturally occurring or intentionally distributed elements. But the seals must also be compatible with the function of the automotive system. They are thus frequently weakened or destroyed through physical stress or chemical contamination and thereby open up vulnerable areas between the pistons and the cylinders for attack by corrosion-inducing agents. When a conducting medium, e.g. water from the road surface, containing mineral salts in solution, is permitted to seep into these areas, an electrochemical reaction is initiated which involves the transfer of ions between the metal surfaces of pistons and cylinder walls and the fluid medium. A localized galvanic current may be produced, for example, by the electrochemical potential difference 'between the electrically positive anode represented by the cylinder wall and the electrically negative cathode represented by the piston surface (or vice versa), which starts the metal-consuming oxidation process dissociating ordinary iron contained in the ferrous material of which the piston may be made, into soluble iron ions (for example, Fe Fe+++2e) thereby corroding the local surfaces.

It is the principal object of my invention to provide an economic and simple protection against piston corrosion which prevents or at least deters deterioration of the pistons.

This object and others which will become apparent hereinafter are attained, in accordance with the present invention, by the introduction of cathodic protection in the form of an auxiliary anode between the conventional dust cap and the piston in a way in which direct contact between the newly inserted anode and the piston surface is established. The direct current flowing from the electrically positive cylinder wall in the direction of the electrically negative piston surface is thus intercepted by the sacrificial metal of the auxiliary anode and prevented from reacting with the piston surface. The interposed electrode, preferentially formed from a metal which is in the negative portion of the electrochemical potential table and is more electronegative than the metal to be protected, therefore substantially reduces or eliminates the destruction of the piston surface by galvanic corrosion. Suitably protective metals are Zinc, magnesium, aluminum and cadmium.

According to the preferred embodiment of my invention the cathodic protection includes a ring made from zinc which gives long-lasting protection against corrosion in the electrochemical environment under consideration. The ring which is inserted in the space provided between the dust cap and a shoulder projecting annularly about the piston, is fitted into an extension of the groove provided in the piston circumference which is the seat for one end of the dust cap.

The above and other objects, features and advantages of my invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is an axial cross-sectional view showing somewhat schematically an overall representation of part of a wheel-brake system including my invention; and

FIG. 2 is an enlarged detail view, partly in section, of the brake.

In the drawing, I show a disk brake for an automotive vehicle whose cantilevered brake yoke half 11 forms a wheel-brake cylinder 2 which is supplied with brake fluid from a master cylinder of the vehicle via the port 12. The other lobe 13 of the housing, which is secured to the unsupported lobe 11 by the bolts 14, is formed with a flange 15 by means of which the brake yoke 16 is attached to the steering knuckle 40 of the motor vehicle wheel 50. The latter may be steered by a conventional tie rod via steering knuckle arm 41. The wheel has a tire carrying disk 51 held by bolts 52 to a hub 53. The yoke 16 extends around the periphery of the brake disk 5 whose axis of rotation is parallel to the axis of the wheelbrake cylinders shown by the line A and perpendicular to the braking faces 5a and 5b of the disk 5. Disk 5 is carried by hub 53 which, in turn, is journaled by bearings 54 and 55 on the shaft tie. Only the unsupported lobe 11 of the yoke 15 and its component elements will be described in detail here, the other side being of similar construction. The piston 1 axially slidable in the cylinder 2 has a contact face 17 designed to apply pressure in the direction of the arrow B to the backing plate 4 of the brakeshoe 18 whose lining 3 confronts the braking face 5a of the disk 5. The piston 1 is slidably engaged by a seal 19 in the wall of the cylinder 2 to prevent brake fluid from escaping from the cylinder. The clip 20 retains the brakeshoe against radial outward movement until it is desired to replace the brakeshoe. The dust-excluding seal 6, designed to prevent entry of contaminants into the wheel-brake cylinder 2, is formed as an annular disk made of flexible material which is attached with its outer rim to the cylinder housing at 7 and has an inner rim hugging the small diameter step 8 of the piston 1, which defines with the large diameter step an annual shoulder along the circumference of the piston. The ring 9, comprising the cathodic protection according to my invention is inserted in space provided between the dust-excluding seal 6 and the shoulder 10 projecting about the piston circumference and is received in an extension of the annular space which is also the seat of the dust-excluding seal 6. Alternate means for inserting the ring 9 in the proximity of the dust-seal attachment may be provided. The zinc ring has been found to substantially completely eliminate galvanic corrosion of the piston when applied to the piston prior to its original insertion in the system.

The invention described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art.

I claim:

1. A cylinder assembly comprising an open-end fluid cylinder, a piston axially slidable in said cylinder and extending into the latter through the open end thereof and having an annular shoulder facing in the direction of said open end, an annular sealing cap anchored on said cylinder and in all-around engagement with said piston at said open end, and a sacrificial cathode-protection ring formed from a metal of more electronegative potential than that of the cylinder and piston mounted on said piston against said shoulder at said open end behind said cap.

2. A hydraulic assembly comprising an open-end fluid cylinder member, a piston member adapted to operate a load through the open end of the cylinder member axially slidable in said cylinder member and having an annular shoulder facing in the direction of said open end, at least one of said members being composed of a galvanically corrodible metal, and a sacrificial cathodic-protection ring formed from a metal of more electronegative potential than said corrodible metal and surrounding said piston member while being seated thereon against said 7 of said cylinder member and engaging said piston member ahead of said ring in the direction of said open end. 4. A cylinder assembly comprising an open-end fluid cylindenmernber; a piston member axially slidable in said cylinder member, at least one of said members being composed of a galvanically corrodible metal; a cathodic-protection ring formed from a metal of more electronegative potential than said corrodible metal and surrounding said piston member; an annular sealing cap seated on the end of said cylinder member and engaging said piston member, said ring being mounted on said piston member adjacent said cap on a side thereof facing away from said open end of said cylinder member, said piston member being formed with a peripheral recess and the rim of said cap and said ring being received in said recess, said piston member forming a shoulder defined by said recess and wherein said ring abuts against said shoulder.

5. The assembly defined in claim 4 wherein said ring is formed from zinc.

6. The assembly defined in claim 4 for a disk brake of an automotive vehicle comprising housing means forming said cylinder member, at least one brakeshoe shiftable by said piston member, and a rotating brake disk engageable by said brakeshoe.

References Cited UNITED STATES PATENTS 1,517,070 11/1924 Jenkins 92-169 2,304,372 12/ 1942 OBannon 92-169 2,994,410 8/1961 Burnett. 3,162,271 12/ 1964 Hanson.

GEORGE E. A. HALVOSA, Primary Examiner US. Cl. X.R. 92224; 188-264 

